1. Characterization of GaAs:Cr Sensors for High Flux X-Ray Imaging Applications
M. C. Veale1, S. J. Bell1,2, D. D. Duarte1,2, M. J. French1, M. Hart1, A. Schneider1, P. Seller1, M. D. Wilson1, G. Georgiou2, P. J. Sellin2, A. D. Lozinskaya3, V. A. Novikov3, O. P. Tolbanov3, A. Tyazhev3 and A. N. Zarubin3
STFC Rutherford Appleton Laboratory, Oxfordshire. OX11 0QX
Faculty of Engineering and Physical Sciences, University of Surrey. GU2 7XH.
Siberian Physical-Technical Institute of Tomsk State University, Tomsk, Russia

2. Outline Why GaAs:Cr? Material Characterization
Initial X-Ray Performance
1mm Thick Devices
Dislocations

3. Large Pixel Detector (LPD)
52cm
LPD detector for XFEL
Based on Hamamatsu Si
500 mm thick sensors
500 mm x 500 mm pixels
Final system 1M pixels
Expected lifetime dose >1MGy.
M. Hart et al. “Development of the LPD, a high dynamic range pixel detector for the European XFEL”. IEEE NSS 2012 ( )

4. Silicon Radiation Damage
A single LPD module
Diamond B16 Beamline
12 keV irradiation
4x1010 photons s-1 mm-2
Pixel currents increased
Radiation damage D A

5. Chromium Compensated GaAs
N-type GaAs
Evaporation of Cr
Annealing
GaAs:Cr
G. I. Azyenshtat t al. NIMA (2001)

6. Materials Characterization
3 Simple Planar devices + Guard Ring.
Ni/GaAs/Ni contacts.
7.0 x 7.0 x 0.5 mm3
Current-Voltage (IV) measurements show quasi-ohmic response. V.
r = 2.4x109 W

7. Electron Transport Alpha particle spectroscopy used to measure mete.
Detectors held under vacuum and bias varied.
Spectra collected for 300s.
tshape = 2 ms.
Count rate ~ 10 cps.
Peak centroids fitted with the SC Hecht equation.
mete ~ 1x10-5 cm2 V-1.

8. Hole Transport Spectra collected for 900s. Count rate ~ 2 cps.
Reduction in count rate despite identical conditions.
Odd behaviour at high voltages.
Reduction in peak centroid with increased voltage?!
mhth <= 5x10-6 cm2 V-1.

9. Initial Spectroscopic Tests
3x 500 mm thick devices.
Assembly of detectors at RAL.
Low-T Ag epoxy bonding.
Bonded to HEXITEC ASIC.
80 x 80 pixels, 250 mm pitch.
Fully spectroscopic readout.
Tested using HEXITEC DAQ.
M. Veale et al. Nuclear Instruments and Methods A, 752, July 2014.

10. 500mm Device Performance 300K Resistivity ~ 3 x 109 W cm
Vbias = V
Leakage current (4cm2) = 2 mA
Electron sensing
= 3.0 +/- 0.5 keV

11. New 1mm Thick Devices

16. Non-Uniformities HEXITEC (250mm) 1mm Thick Medipix (55mm) 0.5mm Thick*
*E. Hamann t al. R07-02

17. Microbeam Line Scans Diamond B16 Beamline. 15keV Monoenergetic X-ray.
Line scans over pixels.
GaAs step size 10 mm.
CdTe step size 25 mm .
Processed using charge sharing discrimination.
Observed 1st, 2nd & 3rd harmonics.

18. Microbeam Line Scans 15 keV counts compared at each scan position.
Variation in counts is evidence for sub-pixel structure.
sCdTe = 1.3 %
sGaAs = 12.2 %
Suggests greater non-uniformity in GaAs:Cr on the 10 mm scale.

19. Field Non-Uniformities
Spectroscopic data used to study non-uniformities.
No variation in FWHM observed!
Only the number of counts per position varies.
Suggestive of lensing effects around dislocations.
Consistent with previous observations in CdTe. B C A

20. Next Steps Strip detectors fabricated 50 mm and 100 mm pitch
512 and 256 channels
Wire bonded to XCHIP ASICs
Readout using Ultra system
Maximum flux = 109 s-1 mm-2
High flux measurements!
100 mm
50 mm

21. Thank You For Listening
Any Questions?

22. No Correction: Highly Uniform Response!
GaAs:Cr Performance
6,400 Pixels
No Correction: Highly Uniform Response!

23. GaAs:Cr Performance
Map of X-Ray Counts

24. GaAs:Cr Performance
Spectroscopic Data Contains Material Information

25. Radiation Tollerance GaAs B16 beam line 108 – 109 photons s-1 mm-2
12 keV mono
8 irradiation areas
Dose rate ~ 10 Gy s-1
Total dose ~ 1 MGy Cu
Slits

26. Flux Calculation (1) (8) Irradiation area varied
Areas: – 3.25 mm2
Leakage current measured
A = mm2
t = 10 s
A = 3.25 mm2
t =10,000 s
s < 0.5 %
Increasing Dose

27. Flux Calculation (1) (8) Irradiation area varied
Areas: – 3.25 mm2
Leakage current measured
Fit to Current Vs. Area
W-Value = 4.2 eV ehp-1
X-ray flux ~ 3 x 108 s-1 mm-2
Si diode ~ 5 x 108 s-1 mm-2
A = mm2
t = 10 s
A = 3.25 mm2
t =10,000 s

28. Dose Calculation Energy = 12 keV l12keV = 11 mm Volume = Area x l
Area = – 3.25 mm2
Flux = 3 x 108 s-1 mm-2
Time = 101 – 105 s
Density = 5.32 g cm-3
Area Doses = 102 – 106 Gy
X-Rays
Dose
𝑫𝒐𝒔𝒆 𝑮𝒚 = 𝑭𝒍𝒖𝒙×𝑨𝒓𝒆𝒂×𝑬𝒏𝒆𝒓𝒈𝒚×𝑻𝒊𝒎𝒆 𝑫𝒆𝒏𝒔𝒊𝒕𝒚×𝑽𝒐𝒍𝒖𝒎𝒆

29. Irradiation Results
TOTAL DOSE ~ 1 MGy
NO CHANGE

30. Flux Calculation - Photons
Cu absorber placed in beam
Spectroscopic data collected
Single photon counting
12 keV peak observed
Small Cu florescence signal
Flux per pixel calculated
X-ray flux ~ 3 x 108 s-1 mm-2